1. Field of the Invention
The present invention relates to a toner for developing electrostatic images used to develop electrostatic latent images in electrophotography, electrostatic printing and other purposes, and more specifically to a toner for developing electrostatic images that has good fixability and offset resistance, and a charge control agent for controlling the amount of charges of the toner.
2. Description of the Prior Art
In copying machines, printers and other instruments based on electrophotography, various toners containing a coloring agent, a fixing resin (binder resin) and other substances are used to visualize (i.e. develop) the electrostatic latent image formed on the photoreceptor having a light-sensitive layer containing an inorganic or organic photoconductive substance. These toners must show satisfactory performance as to chargeability, fixability, offset resistance, etc.
Chargeability is a key factor in electrostatic latent image-developing systems. Thus, to control appropriately toner chargeability, a charge control agent providing a positive or negative charge is often added to the toner.
Of the conventional charge control agents in actual application or use, those providing a positive charge for the toner include nigrosine dyes and quaternary ammonium salt compounds. Charge control agents providing a negative charge for the toner include chromium complexes of azo dyes and metal complexes (metal salts) of alkyl salicylic acid and hydroxy naphthoic acid.
However, dye type charge control agents lack versatility for use in color toners, although providing good chargeability. Quaternary ammonium salt type charge control agents generally lack environmental resistance under high-temperature high-humidity conditions. Metal complexes or salts with an aromatic hydroxy carboxylic acid or the like as a ligand are inferior to dye type charge control agents in their dispersibility in resin (e.g. toner binder resin).
In recent years, to improve image quality while increasing copying and printing speeds, there has been an increasing demand regarding toner charge properties, such as increased rise speed, and toner fixing properties on recording paper, such as good low-temperature fixability and offset resistance.
However, not a few such conventional charge control agents affect adversely the thermal melting property of the binder resin in the toner, resulting in decreased fixing performance.
There are a number of known toners supplemented with various phenol compounds for the purpose of improving toner fixability and the long-run property or preventing surface-treated carrier deterioration.
For example, Unexamined Japanese Patent Publication No. 138357/1988 discloses a toner containing an oligomer of a phenol compound having an alkyl-substituted amino group. Unexamined Japanese Patent Publication No. 291569/1990 discloses a toner incorporating a compound having a phenolic OH group and a xanthene dye. Unexamined Japanese Patent Publication No. 266462/1988 discloses toners containing a compound such as 2,6-di-tertiary-butyl-4-ethylphenol or 2,2'-methylene-bis-(4-methyl-6-tertiary-butylphenol). However, the developing agents incorporating these toners are unsatisfactory in charging properties.